Delay locked loops (DLLs) are often used in integrated circuits (ICs) to generate an internal clock signal. The internal clock signal is generated by applying a delay to a system clock or an external clock signal. The DLL automatically adjusts the delay to keep the internal and external clock signals synchronized.
A conventional DLL includes a delay line. The delay line includes a number of delay stages to apply an amount of delay to an external clock signal to generate an internal clock signal. A phase detector compares the phase difference between the external and internal clock signals. If the external and internal signals are not synchronized, the phase detector generates shifting signals. A shift register connected to the delay lines receives the shifting signals to adjust the amount of delay applied to the external clock signal. The phase comparison and delay adjustment process repeats until the external and internal and clock signals are synchronized. When the external and internal and clock signals are synchronized, the phase detector disables the shifting signal to lock the DLL.
In each delay adjustment in a conventional DLL, the delay applied by the delay line is increased or decreased by a delay of one delay stage of the delay line. Thus, after each delay adjustment, the phase difference between the external and internal clock signals become narrower. In other words, after each delay adjustment, the internal clock signal comes closer to an allowable window that indicates the synchronism between the external and internal clock signals. In some cases, the internal signal may miss the window, i.e., overshoot or undershoot the target delay, and therefore a subsequent adjustment step is unnecessarily created. Moreover, adjusting the delay by a delay stage of the delay line may not provide satisfactory level of accuracy especially if the DLL is used in new generations of high speed devices such as microprocessor and dynamic random access memory (DRAM) devices.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an improved DLL.